1. Technical Field
The present invention relates to hearing evaluation and hearing aid fitting. More particularly, the present invention relates to virtual electroacoustic audiometry for unaided, simulated aided, and aided hearing evaluation.
2. Description of the Prior Art
The human auditory system processes sounds from a complex three-dimensional space via the external, middle, and inner ear, as well as via the complex neural pathways that lead to the auditory cortex within the brain. A measurable hearing loss, due to various conductive, sensorineural, or central auditory disorders, affects a significant percentage of the human population, particularly elderly persons. Rehabilitation via hearing aids remains the only viable option for those types of hearing impairments that cannot otherwise be medically treated or surgically alleviated.
Advances in hearing aids and fitting technologies are continuously being made. Today's ear-level hearing aids, i.e. in-the-ear (ITE), behind-the-ear (BTE), in-the-canal (ITC), and completely-in-the-canal (CIC) types, are more cosmetically appealing due to improvements in electronic and mechanical miniaturization. More significant, however, is the increasing availability of advanced hearing aid signal processing schemes, such as adaptive filtering and multi-band dynamic compression.
As manufacturers are continuously developing new hearing aids with unique signal processing schemes, a hearing aid dispensing professional is faced with the increasingly difficult task of prescribing and selecting a hearing aid for a hearing-impaired individual from the available selection. A cursory look at available hearing aid processing schemes reveals an impressive array of categories, sub-categories, and associated acronyms that are baffling to most hearing aid dispensing professionals (see Mueller, H. G., A Practical Guide To Today's Bonanza of Underused High-Tech Hearing Products, The Hearing Journal, vol. 46, no. 3, pp. 13-27, 1993).
Today, optimal fitting of prescription hearing aids remains an elusive goal in auditory rehabilitation. The fundamental problem is that there are numerous electrical, acoustic, physical, and other parameters that affect hearing aid performance. These parameters include signal processing schemes, electronic circuit adjustments, size of hearing aid, insertion depth, venting size, patient controls, and life-style related factors that must be considered when prescribing and fitting a hearing aid. These hearing aid parameters are not only complex and highly interrelated, but also vary according to the unique interaction of the hearing device with the hearing-impaired individual.
Generally, the in situ performance characteristics of a hearing aid cannot be predicted with today's conventional fitting instrumentation and methods. Dissatisfaction among hearing aid user's, partially due to poor hearing aid prescription fitting, is manifested by a high return rates, often exceeding 20% according to industry reports.
Factors that Contribute to Unsatisfactory Hearing Aid Results
I. Inaccuracy of conventional diagnostic audiometry
Assessment of hearing is the first step in the prescribing and fitting of a hearing aid. Accurate assessment of the individual's hearing function is important because all hearing aid prescriptive formulas depend on one or more sets of hearing diagnostic data (see Mueller, H. G., Hawkins, D. B., Northern, J. L., Probe Microphone Measurements: Hearing Aid Selection and Assessment, Singular Publishing Group, Inc., 1992: Ch. 5).
The hearing aid prescription process involves translating the diagnostic data into target hearing aid electroacoustic parameters that are used in the selection of the hearing aid. Traditional hearing evaluation methods and instruments employ a variety of air-conduction transducers for coupling acoustic signals into the ear. Commonly used transducers include supra-aural earphones, such as TDH-39, TDH-49, TDH-50, insert earphones, such as ER-3A, and free-field speakers (see Specification of Audiometers, ANSI-S3.6-1989, American Standards National Institute).
A threshold measurement obtained with such transducers is referenced to a mean threshold obtained by testing a group of otologically normal individuals. This mean threshold, by definition, is referred to as the zero decibel hearing-level or 0 dB HL. With this zero reference concept, threshold measurements of otologically normal persons can vary by 20 dB or more. These variations can be attributed to following factors:
1. Variability due to transducer type used and placement with respect to the ear.
In a study by Mowrer, et al discrepancies of 10 dB were found in 36% of threshold measurements (see Mowrer, D. E., Stearns, C., Threshold measurement variability among hearing aid dispensers, Hearing Instrument, vol. 43, No. 4, 1992). Another major disadvantage of measurements obtained using a traditional transducer is that results are not interchangeable with measurements taken with another transducer for a given individual (see Gauthier, E. A., Rapisadri, D. A., A Threshold is a Threshold is a Threshold . . . or is it?, Hearing Instruments, vol. 43, no. 3, 1992).
2. Variability due to transducer calibration methods that employ couplers that do not represent the human ear.
Although recently developed couplers more closely match the acoustic impedance characteristics of an average human ear, there is still disagreement as to the accuracy of this artificial ear (see Katz, J., Handbook of Clinical Audiology, Third Edition, 1985, pp. 126). Most calibration methods today rely on 6-cc or 2-cc couplers that are known to have considerable acoustic characteristic discrepancies from real human ears (see Specification of Audiometers, ANSI-S3.6-1989, American Standards National Institute). Furthermore, even if an agreement was made regarding an average artificial ear, variability among individuals is significant due to individual acoustic characteristics of pinna, ear canal, concha, and to a lesser extent, the head, and the torso (see Mueller, H. G., Hawkins, D. B., Northern, J. L., Probe Microphone Measurements: Hearing Aid Selection and Assessment, 1992, pp. 49-50). In one study, inter-subject variability was up to 38 dB across six standard audiometric frequencies when sound pressure levels (SPL) were measured at the tympanic membrane for 50 ears of 25 adults (see Valente, M., Potts, L., Valente, M., Vass, B., Intersubject Variability of Real-Ear SPL:TDH-39P vs ER-3A Earphones, In press, JASA).
3. Conventional audiometric measurement methods do not provide a means of self-calibration even though transducer characteristics are known for changes due to wear or damage of the moving diaphragm.
Clinicians who use regular subjective listening methods simply cannot detect gradual changes in transducer sensitivity.
Although errors due to the above factors are not likely to be accumulative in all cases, the potential for substantial errors is always present. Furthermore, these errors are not consistent across all frequencies and therefore cannot be simply compensated for during the fitting process via an overall volume adjustment.
II. Lack of realistic listening conditions in the unaided and aided hearing assessment
1. Lack of Binaural Advantage Considerations.
Many studies have demonstrated the advantage of binaural versus monaural listening (see Cherry, E. C., Some Experiments on the Recognition of Speech with One and Two Ears, JASA, vol. 25, no. 5, 1953, pp. 975-979; Cherry, E. C., and Tylor, W, K., Some Further Experiments on the Recognition of Speech with One and Two Ears, JASA, vol. 26, 1954, pp. 549-554). These studies have focused on the advantages offered by the Binaural Masking Level Difference (BMLD) and Binaural Intelligibility Level Difference (BILD).
Early studies of BMLD and BILD involved the presentation of signal and noise to one or both ears at various phase relationships. Tone detection and speech intelligibility were shown to vary as much as 15 dB, depending on the signal/noise phase relationship. Even though many of these studies suggest the significance of binaural considerations, today's hearing assessment methods, unaided and aided, primarily deal with monaural test conditions, i.e. testing one ear at a time.
2. Lack of Spatialized Sound Considerations.
When audiometric signals such as speech and/or noise are delivered to the ear via a conventional audiometers and associated transducers, the sound perception by the test subject is not localized to any particular point in space (see Specification of Audiometers, ANSI-S3.6-1989, American Standards National Institute). For example, in speech audiometry evaluation, the speech stimuli level is adjusted for one ear and speech noise level is separately adjusted in the opposite ear. The test subject perceives sounds to be within the head and localization is limited to left/right direction. This type of signal presentation and perception is referred to as intracranial and is unlike the way humans normally perceive natural sounds. Recent studies by Bronkhorst and Plomp, and Begault expanded on previous binaural interaction advantage studies by employing headphone localization techniques (see Bronkhorst, A. W., Plomp, R., The Effects of Head-Induced Interaural Time and Level Differences on Speech Intelligibility in Noise, Journal of the Acoustical Society of America, vol. 83, no. 4, 1988, pp. 1508-1516; Bronkhorst, A. W.; Plomp, R., The Effects of Multiple Speech-like Maskers on Binaural Speech Recognition in Normal and Impaired Hearing, Journal of the Acoustical Society of America, vol. vol. 92, no. 6, 1992, pp. 3132-3139; and Bagault, D. R., Call Sign Intelligibility Improvement Using a Spatial Auditory Display, Ames Research Center, NASA Technical Memorandum 104014, April 1993). The results of these studies conclude the speech perception is not only dependent on intensity levels but also on the spatial relationship between speech and noise.
3. Lack of Evaluation Methods in Realistic Listening Environments.
Speech intelligibility and discrimination deteriorates in the presence of competing speech and other environmental sounds. Furthermore, the acoustic properties of a room, e.g. its walls and objects within the room, all play an important role in the filtering process subjected to the original signal source. These filtering effects are especially significant for hearing-impaired individuals who typically have a limited frequency response and dynamic range in their hearing function.
Today's methods of presenting competing and environmental sounds via conventional transducers fail to represent the acoustic reality of the typical listening condition. Recorded sound material presented via tape players, compact disks, or computer digital playback are subject to filtering effects of the transducer employed and/or the room acoustics of the clinical setup. There are no hearing assessment methods today that can evaluate or predict the hearing performance of an individual in a specific and realistic listening scenario.
For example, the hearing performance of a hearing-impaired child in a typical classroom in the unaided condition, and the hearing performance of the child with a specific hearing aid, i.e. aided hearing, in the same classroom environment. These and other auditory experiences are presently considered a fact of life that can not be dealt with in a clinical setup (see Mueller, H. G., Hawkins, D. B., Northern, J. L, Probe Microphone Measurements: Hearing Aid Selection and Assessment, 1992, pp. 69).
III. Limitations of current real-ear measurement (REM) equipment and methods
In recent years, real ear measurement (REM) systems were developed to assess the in situ performance of a hearing aid. REM consists of test probe measurements of the ear response to free field stimulus, i.e. speakers, taken at the tympanic membrane. A secondary reference microphone is typically placed outside the ear canal close to the ear canal opening. The reference microphone is used to calibrate the test probe as well as to regulate the stimulus level as the head moves with respect to the free field speaker.
For a comprehensive REM evaluation, measurement of the real ear response for the unaided, i.e. open canal, condition is first taken. Target hearing aid characteristics are then calculated based on the natural ear canal response characteristics, as well as other criteria (see Mueller, H. G., Hawkins, D. B., Northern, J. L., Probe Microphone Measurements: Hearing Aid Selection and Assessment, 1992, Ch. 5). When the hearing aid is prescribed, ordered, and received during a subsequent visit, the aid is inserted over the probe tube and adjusted to match the prescribed target hearing aid characteristics.
REM evaluation and REM-based prescriptive methods provide considerable improvements over previous fitting methods which relied on the combination of audiometric data and hearing aid 2-cc coupler specifications. Although REM offers insight into the in situ performance of the hearing aid, it suffers from several fundamental problems, as described below:
1. REM test results vary considerably depending on speaker position/orientation with respect to the ear, particularly at higher frequencies (see Mueller, H. G., Hawkins, D. B., Northern, J. L., Probe Microphone Measurements: Hearing Aid Selection and Assessment, 1992, pp. 72-74).
2. Real ear measurements are taken with a specific stimulus type, source-ear distance/orientation, and room acoustics. The specific test condition may not represent realistic listening scenarios encountered by hearing aid users. In fact, using conventional REM approaches, a hearing aid may be optimized for a specific listening condition while compromising the performance under other conditions that may be more important to the hearing-impaired individual.
3. Accurate REMs require careful placement of the test probe within the ear canal of an individual. The closer the probe to the tympanic membrane, the more accurate the results are, particularly for high frequency measurements (see Mueller, H. G., Hawkins, D. B., Northern, J. L., Probe Microphone Measurements: Hearing Aid Selection and Assessment, 1992, pp. 74-79).
Present methods of probe placement are highly dependent on the operating clinician's skill and the specific length of the canal, which is about 25 mm for the average adult. Today's REM methods rely on visual observation of the probe tip. This is especially problematic when a hearing aid is placed in the canal during the aided evaluation process. The only exception to the conventional visual method is the acoustic response method developed by Nicolet Corp. for use in the Aurora system (see Chan, J., Geisler, C., Estimation of Eardrum Acoustic Pressure and Ear Canal Length from Remote Points in the Canal, J. Acoust. Soc. Am. 87 (3), March 1990, pp. 1237-1247; and U.S. Pat. No. 4,809,708, Method and Apparatus for Real Ear Measurements, March 1989). However, Nicolet's acoustic response method requires two calibration measurements prior to placement of the probe at the desired position within the ear canal.
4. REM test results vary considerably depending on the placement of the reference microphone near the ear. The errors are especially significant at frequencies of 6 kHz and higher (see Mueller, H. G., Hawkins, D. B., Northern, J. L., Probe Microphone Measurements: Hearing Aid Selection and Assessment, 1992, pp. 72-74).
5. REM instruments employ sound field speakers in a room with ambient background noise that often exceeds 50 dB SPL across standard audiometric frequencies. This necessitates stimulus levels of 60 dB or higher to produce measurements having sufficient signal-to-noise ratios. This is problematic if hearing aid performance characterization under low level acoustic stimuli is required.
IV. The problem of correlating diagnostic, prescription formulae, and real ear measurements
A significant factor that contributes to the results of a hearing aid fitting is the problem of adequately correlating diagnostic data with fitting needs of the hearing-impaired individual. Diagnostic measurements are typically taken in dB HL with transducers that are calibrated in 6-cc couplers. Hearing aid specification and performance measurements employ 2-cc couplers which do not represent the real-ear. Fitting involves the use of one of several prescriptive formulae, with results that are known to vary as much as 15 dB for the same diagnostic data across standard audiometric frequencies (see Mueller, H. G., Hawkins, D. B., Northern, J. L., Probe Microphone Measurements: Hearing Aid Selection and Assessment, 1992, p 107). These fitting formulae incorporate statistically based conversion factors that simplify the correlation of hearing aid requirements to a particular hearing impairment. However, averaged conversion factors are known to vary considerably with respect to objectively measured individual conversion factors.
Several methods and protocols have been suggested to alleviate errors associated with measurement errors and data correlation (see Sandberg, R., McSpaden, J., Allen, D., Real Measurement from Real Ear Equipment. Hearing Instruments, Vol. 42, No. 3, 1991, pp. 17-18). However, many of these protocols have not yet been widely accepted due to limitations of conventional audiometry and Real-Ear Measurement (REM) equipment and other factors related to efficiency of the proposed protocols in clinical setups.
Hearing rehabilitation through the use of hearing aids remains the only viable option for many hearing impaired individuals who cannot be medically or otherwise treated. A full audiometric evaluation is a required first step prior to fitting a hearing aid. Pure tones and one or more speech perception tests are typically involved in the basic audiometric test battery. Suprathreshold measurements may also be taken to establish a hearing dynamic range profile, in addition to the frequency response profile obtained in the threshold audiogram test. Following the audiometric evaluation, a hearing aid is then prescribed, selected, ordered, and subsequently tried and adjusted after being received from the manufacturer or assembled in the clinic. The fitting or determination of the electroacoustic parameters of a hearing aid typically involve a combination of objective measurements to achieve a desired target characteristics based on one of many prescriptive formulae and subjective measures based on the individual's subjective response to speech and other sounds at various loudness levels.
Conventional audiometry methods, employing headphones, inserts, or sound-field speakers, rely on presenting acoustic energy to the ear of the individual in a manner which is not representative of sound delivery under realistic listening conditions. Conventional audiometers present various tones, speech, and noise stimuli to each ear individually and thus are not capable of investigating the individual's binaural integration advantage, or of assessing the hearing function in a three-dimensional sound environment.
Another major disadvantage of conventional audiometry methods is the inability of such methods to assess accurately and objectively, in absolute physical terms such as dB SPL, the hearing function of an individual with respect to the inside of the ear canal to correlate unaided evaluation results to hearing aid requirements. One exception is the probe-mike-calibrated fitting system developed by Ensoniq, which only addresses testing accuracy (see Gauthier, E. A., Rapisadri, D. A., A Threshold is a Threshold is a Threshold . . . or is it?: Hearing Instruments, vol. 43, no. 3, 1992).
Furthermore, conventional audiometry instruments and methods are not capable of simulating the electroacoustic performance of one or more prescribed hearing aids and assessing their simulated function in realistic acoustic conditions relevant to the individual's unique listening requirements.
The master hearing aid concept, which gained some popularity in the '70s and '80s, involves an instrument that presents simulated hearing aids to the hearing aid user (see Selection Instrumentation/Master Hearing Aids in Review, Hearing Instruments, Vol. 39, No. 3, 1988). Veroba et al (U.S. Pat. No. 4,759,070, Patient Controlled Master Hearing Aid, Jul. 19, 1988) describe a patient controlled hearing aid module that is inserted into the ear canal and connected to a test module which offers multiple signal processing options, e.g. analog circuit blocks, to the individual. Hearing aid characteristics are determined by a tournament process of elimination, while the hearing-impaired person is presented with real-world sounds played back from tape decks via a set of speakers located around the hearing-impaired person's head. The system's fitting process is based on subjective responses of the hearing-impaired who must continuously decide on an alternative signal processing option, and supposedly eventually arrive at an optimal fitting.
The fitting process via the Veroba system, commercially known as the Programmable Auditory Comparator, an essentially obsolete product, does not involve any objective measurements or calculations for selecting and fitting of the hearing aid. In fact, the entire fitting process is based on the subjective response of the hearing impaired person. Clearly, most hearing impaired individuals, on their own, cannot explore in a timely and efficient manner the spectrum of various complex and interrelated electroacoustic parameters of a hearing aid under various listening environments. A serious limitation of Veroba is that it does not teach how to assess objectively the performance of the simulated hearing aid, nor does it teach how the aided performance is related to the individual's unaided response determined previously during the audiometric evaluation process.
A major unsubstantiated claim in Veroba's system is the simulation of a realistic acoustical environment via tape-deck playback and speakers located around the head of the hearing-impaired individual. However, recorded acoustic signals that are played back are further subjected to acoustic modifications due to speaker characteristics, speaker position with respect to ear/head, and acoustic characteristics of the room, i.e. wall reflections and acoustic absorption. Without factoring in all of the specific acoustic modifiers in the transmission channel between the tape-deck and the individual's ear, a realistic listening condition cannot be achieved with Veroba or any such system. Furthermore, Veroba is not capable of manipulating the acoustic condition from its recorded form, e.g. by projecting an audio source in a specific location within a three-dimensional acoustic space with a specific acoustic boundary condition.
Another hearing aid simulator, the ITS-hearing aid simulator developed by Breakthrough, Inc. offers computer digital audio playback of digital recordings obtained from the output of various hearing aids (see ITS-Hearing Aid Simulator, Product brochure, Breakthrough, Inc., 1993). Each recording segment represents a specific acoustic input, listening scenario, hearing aid model, and hearing aid electroacoustic setting. The recording segments require memory space either on a hard disk or other known forms of memory storage devices, such as compact-disk read-only-memory. This digital-recording-based approach renders impractical the arbitrary selection of a hearing aid, hearing aid setting, and input stimulus for a hearing-impaired individual, when considering all the possible combinations. Furthermore, the effects of hearing aid vent sizes, and associated occlusion effect, insertion depth, and individual external ears, cannot be simulated with the proposed hearing aid simulator because it relies on conventional transducers, i.e. headphones and insert earphones.
For similar reasons, many other commercially available master hearing aid systems, do not have the ability to simulate accurately a hearing aid in a realistic listening environment. Furthermore, these systems do not include objective measurement methods for evaluating simulated aided versus unaided conditions. For these and other reasons, virtually all dispensed hearing aids today are fitted without the use of master hearing aid or hearing aid simulator instruments.
State-of-the-art REM equipment allows for in-the-ear-canal acoustic response measurements. The acoustic stimuli are typically generated by the REM equipment itself and delivered via a speaker, typically positioned at 0.degree. azimuth, or with two speakers positioned at 45.degree. azimuth, with the respect to the transverse plane of the head. The response measurements, i.e. free-field to real-ear transfer function, are essentially one-dimensional since they only provide a single transfer function per ear in a particular speaker-ear relationship, and are thus not capable of establishing a multi-dimensional profile of the real-ear response. Another disadvantage of conventional REM equipment and methods is the lack of real speech stimuli presentation because most REM equipment only offer pure-tone, pure-tone sweep, speech-noise and other speech-like stimuli. These stimuli do not explore responses to particular speech segments that may be important to the hearing-impaired individual during unaided and aided conditions.
Recent developments relating to electroacoustic hearing aid measures involve the testing of hearing aids in more realistic conditions. Real speech signals instead of pure tones and speech-like noise signals were employed in a recommended test protocol; and spectrogram plots indicating temporal, i.e. time, analysis of the acoustic energy in dB SPL versus frequency was compared for hearing aid input versus output (see Jamieson, D., Consumer-Based Electroacoustic Hearing Aid Measures, JSLPA Suppl. 1, January 1993). The limitations of the proposed protocol include: limited acoustic reality due to the specified sound delivery method via a speaker to a hearing aid in an enclosed chamber; and limited value of the spectrogram plots which do not directly indicate the relationship of the plot to audibility and loudness discomfort.
Other recent developments involve three-dimensional sound presentation via headphone transducers (see Wightman, F. L., Kistler, D. J., Headphone Simulation of Free-Field Listening. I: Stimulus Synthesis, JASA. vol. 85, no. 2, 1989, pp. 858-867; and Wightman, F. L., Kistler, D. J., Headphone Simulation of Free-Field Listening. II: Psychophysical Validation, JASA. vol. 85, no. 2, 1989, pp. 868-878). These three-dimensional effects are achieved by recreating the in-the-ear-canal acoustic response to free-field signals via headphones or speakers (see U.S. Pat. No. 4,118,599, Stereophonic Sound Reproduction System, Oct. 3, 1978; U.S. Pat. No. 4,219,696, Sound Image Localization Control System, Aug. 26, 1980; U.S. Pat. No. 5,173,944, Head Related Transfer Function Pseudo-Stereophony, Dec. 22, 1992; U.S. Pat. No. 4,139,728, Signal Processing Circuit, Feb. 13, 1979; and U.S. Pat. No. 4,774,515, Altitude Indicator, Sep. 27, 1988). This involves digital filtering of source signals based on head-related-transfer-function (HRTF). The HRTF, essentially real-ear unaided response (REUR) in three-dimensional space, is a frequency dependent amplitude and time delay measurement that results from head shadowing, pinna, concha, and ear canals. The HRTF enables externalization of localized sound with headphones. Source signals that are processed with HRTF provide the listener with free-field listening experience according to the controls of the signal processing parameters.
Present research and development efforts in three-dimensional audio is mainly focused on commercial musical recordings, playback enhancement, and human-machine interface enhancement (see Bagault, D. R., Call Sign Intelligibility Improvement Using a Spatial Auditory Disaply, Ames Research Center, NASA Technical Memorandum 104014, April 1993; and Begault, D., Wenzel, E., Headphone Localization of Speech, Human Factors, 25 (2), pp. 361-376, 1993) and virtual reality systems (see The Beachtron-Three-dimensional audio for PC-compatibles, reference manual, Crystal River Engineering, Inc., Revision D, November, 1993). The object of these three-dimensional audio systems has been limited to simulating situational awareness in an approximate virtual acoustic environment since non-individualized HRTF set is typically employed.
The application of three-dimensional audio in objective in-the-ear-canal assessment of hearing in the unaided, simulated aided, and aided conditions would be a significant and extremely helpful departure from known audiometric techniques.